Open the simulator using Xcode (click on the Xcode menu > Open Developer Tool > iOS Simulator) and you should be able to see and use the app in the simulator.

You can also use the Application 'Terminal' to install the app. (Needed for running on the iPad Simulator)

Hit Cmd + Spacebar and type Terminal

Open up Xcode and Launch the iOS Simulator by clicking on the dropdown Xcode > Open Developer Tool > iOS Simulator.

Go back to Terminal and Run the following command to install the app to the simulator which you just opened.(replace <location of .app file> and drag and drop the simulator build you downloaded into the terminal window. Make sure you unzip the download and are dragging the file with the .app extension on the end)